In a cellular network, user equipment (UE) perform cell search and cell identification for the purpose of mobility, radio resource management (RRM), etc. Cell search and cell identification may also be performed by a radio network node equipped with the corresponding interface, e.g., a femto eNodeB operating in a listening mode. In a more general case, not restricted to cellular networks, a wireless device or a radio node performs a search of neighbour wireless devices or radio nodes, e.g., a Bluetooth device, a laptop with a radio interface activated, another mobile, an iPhone, a WiFi node, neighbour radio nodes, etc. Accordingly, identification of a radio node or cell may be performed in a cellular network, an ad hoc network, a sensor network, a WiFi network, etc.
From the perspective of a UE in a 3GPP cellular network, a UE finds a cell by receiving radio signals and searching for signals with a specific signature known to the UE. To identify a new cell, the UE has to identify the cell (e.g., with a non-globally unique identity) and then, optionally or upon a request, obtain a globally unique Cell Global Identity (CGI).
In Long Term Evolution (LTE) networks, cell identification includes detection of a cell and then additionally performing verification. For example, a UE detects a specific physical-layer cell identity (PCI) and then verifies that the specific PCI was actually detected, as opposed to being detected in error.
Cell detection in LTE is performed based on synchronization signals; specifically, based on a Primary Synchronization Signal (PSS) and a Secondary Synchronization Signal (SSS). The synchronization signals occupy 62 resource elements in the center of the allocated bandwidth as shown in FIGS. 1 and 2. In a synchronous network, PSS/SSS from one cell overlap/interferer with PSS/SSS from another cell, which correspond to reuse-1 or 100% load all the time on these signals.
Unique combinations of PSS and SSS provide 504 unique PCIs, which may be reused in the same PLMN network on one frequency and/or across frequencies. These PCIs are grouped into 168 unique physical-layer cell-identity groups, each group containing three unique identities. The grouping is such that each physical-layer cell identity is part of one and only one physical-layer cell-identity group. A physical-layer cell identity NIDcell=3NID(1)+NID(2) is thus uniquely defined by a number NID(1) in the range of 0 to 167, representing the physical-layer cell-identity group, and a number NID(2) in the range of 0 to 2, representing the physical-layer identity within the physical-layer cell-identity group. The cell identity group is determined based on the known SSS sequences, and the identity within the group is determined based on the known PSS sequences.
The PCI of a detected cell can then be used to determine sequences of other signals (e.g., cell-specific reference signals, CRS, positioning reference signals, PRS, etc.) and their allocation in the time-frequency grid. The CRS signals in LTE are allocated in the time-frequency grid as shown in FIG. 3. Different cells can use 6 different shifts in frequency. In practice, there is a reuse-6 pattern for CRS transmitted from 1 TX antenna ports and reuse-3 pattern for CRS transmitted from 2 TX antenna ports.
With the CRS determined in this way, the detected cell is verified by performing a signal strength measurement (e.g., a reference signal received power (RSRP) measurement) on that CRS.
After identifying the cell, the UE may be requested by the associated eNodeB to report the CGI of the cell. The request may be triggered by the eNodeB receiving a measurement report for the identified cell from the UE. That is, the CGI reading request may follow the reporting of the newly identified cell (where the newly identified cell is detected and verified by RSRP measurement). Regardless, the CGI may be obtained via reading system information transmitted over a broadcast channel.
Consider now cell identification in the context of a multi-carrier system that permits carrier aggregation (CA). A multi-carrier system (or interchangeably called as the CA) allows the UE to simultaneously receive and/or transmit data over more than one carrier frequency. Each carrier frequency is often referred to as a component carrier (CC) or simply a serving cell in the serving sector, more specifically a primary serving cell (PCell) or secondary serving cell (SCell). The multi-carrier concept is used in both High Speed Packet Access (HSPA) and LTE. Carrier aggregation is supported for both contiguous and non-contiguous component carriers, and component carriers originating from the same eNodeB need not to provide the same coverage.
Serving Cell: For a UE in RRC_CONNECTED not configured with CA there is only one serving cell comprising of the primary cell (PCell). For a UE in RRC_CONNECTED configured with CA the term ‘serving cells’ is used to denote the set of one or more cells comprising of the primary cell and all secondary cells.
Primary Cell (PCell): the cell, operating on the primary frequency, in which the UE either performs the initial connection establishment procedure or initiates the connection re-establishment procedure, or the cell indicated as the primary cell in the handover procedure.
Secondary Cell (SCell): a cell, operating on a secondary frequency, which may be configured once an RRC connection is established and which may be used to provide additional radio resources.
In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC). Depending on UE capabilities, Secondary Cells (SCells) can be configured to form together with the PCell a set of serving cells. In the downlink, the carrier corresponding to an SCell is a Downlink Secondary Component Carrier (DL SCC) while in the uplink it is an Uplink Secondary Component Carrier (UL SCC).
Activation and deactivation of secondary cells: In CA, the base station (e.g. eNode B) in LTE can deactivate one or more secondary cells on the corresponding secondary carriers. The deactivation is done by the eNB using lower layer signalling (e.g. over PDCCH in LTE) using a short command such as ON/OFF (e.g. using 1 bit for each SCell). The activation/deactivation command is sent to the UE via the PCell. Typically the deactivation is done when there is no data to transmit on the SCell(s). The activation/deactivation can be done independently on uplink and downlink SCell. The purpose of the deactivation is thus to enable UE battery saving. The deactivated SCell(s) can be activated also by the same lower layer signalling.
In this context, cell identification may generally be performed via intra-frequency measurements (i.e., measurements on the frequency of a serving cell, whether PCell or SCell), inter-frequency (i.e., measurements on a frequency different than that of a serving cell), or inter-RAT measurements (i.e., measurements on a radio access technology, RAT, different than that of a serving cell). Inter-frequency and inter-RAT measurements may also be inter-band when the frequencies belong to different frequency bands.
In this context, the non-CA UE would normally require measurement gaps for performing inter-frequency or inter-RAT cell identification. The same applies for CA-capable UE when performing cell identification on non-configured or deactivated carrier. CA-capable UE would, however, normally not require measurement gaps for measurements on SCC. There is also an on-going discussion in 3GPP on non-CA capable UE which are capable of performing measurements without measurement gaps. Thus, a UE may not require measurement gaps for performing measurements on a configured carrier component.
Cell identification may also be performed during specifically configured low-interference time periods, e.g., indicated by a time-domain measurement resource restriction pattern which the network may signal to the UE to facilitate enhanced Inter-Cell Interference Coordination (eICIC) in heterogeneous deployments. It is, however, noted that such measurement patterns do not help to improve the interference situation on PSS/SSS in a synchronous network or frame-aligned network where PSS/SSS always experience 100% load since these signals are always transmitted in all cells. The patterns may, however, be useful for improving RSRP accuracy or RSRQ level which may differ in different subframes and thus the network may indicate to the measuring UE the subframes with the preferred interference conditions.
Also note that the current cell identification requirements specify a certain period T during which the UE has to perform cell identification and report a corresponding event to the network. The required period T includes both the time necessary for detecting a cell and the time T1 for performing a measurement. The current standard specifies both T and T1 time periods. Further, the UE is typically required to report N (e.g., N=8) identified cells within the required period. The requirements for cell identification typically differ (e.g., in the measurement period length, number of cells, number of frequencies, etc.) for intra-frequency, inter-frequency, and inter-RAT.
Cell identification proves similar in networks other than LTE. In WCDMA, for example, the UE detects a cell using signals transmitted on primary and secondary synchronization channel, as well as CPICH scrambling code detection. The UE then verifies the cell using signal strength measurements, called Received Signal Code Power (RSCP). In GSM, the UE reads a broadcast channel (BSIC) as “cell search” (the UE knows the frequency, so it only needs to find the broadcast channel), performs a received-signal level measurement, RXlev (RSSI), which is then reported.
In other systems, such as those based on machine-to-machine (M2M) and device-to-device (D2D) communication, identification of other radio nodes or devices also proceeds in a similar fashion. For example, in D2D communication there is also some sort of device discovery and device verification. In network-assisted D2D, the network gives sufficient information to a slave in order to be able to detect a master, for instance the beacon signal to search for, time-frequency resources to find the beacon, etc. The verification is then mainly done by determining the reliability of the detected signal. If it is reliable that the specific beacon has been detected, then it has been verified. Sometimes the verification information is also fed back to the network node (including for instance received beacon power level, etc.) for further D2D mode selection by the network node.
Accordingly, whether in the context of node/cell identification or in some other context, verification in a wireless communication system has heretofore exclusively relied upon the result of a power-based measurement.